Transition metal hexacyanoferrate battery with single plateau charge/discharge curve

ABSTRACT

A transition metal hexacyanoferrate (TMH) cathode battery is provided. The battery has a A x Mn[Fe(CN) 6 ] y .zH 2 O cathode, where the A cations are either alkali or alkaline-earth cations, such as sodium or potassium, where x is in the range of 1 to 2, where y is in the range of 0.5 to 1, and where z is in the range of 0 to 3.5. The A x Mn[Fe(CN) 6 ] y .zH 2 O has a rhombohedral crystal structure with Mn 2+/3+  and Fe 2+/3+  having the same reduction/oxidation potential. The battery also has an electrolyte, and anode made of an A metal, an A composite, or a material that can host A atoms. The battery has a single plateau charging curve, where a single plateau charging curve is defined as a constant charging voltage slope between 15% and 85% battery charge capacity. Fabrication methods are also provided.

RELATED APPLICATIONS

This application is a Divisional of an application entitled, TRANSITION METAL HEXACYANOFERRATE BATTERY CATHODE WITH SINGLE PLATEAU CHARGE/DISCHARGE CURVE, invented by Yuhao Lu et al., Ser. No. 13/752,930, filed Jan. 29, 2013;

which is a Continuation-in-Part of an application entitled, SUPERCAPACITOR WITH HEXACYANOMETALLATE CATHODE, ACTIVATED CARBON ANODE, AND AQUEOUS ELECTROLYTE, invented by Yuhao Lu et al., Ser. No. 13/603,322, filed Sep. 4, 2012;

This application is a Continuation-in-Part of an application entitled, IMPROVEMENT OF ELECTRON TRANSPORT IN HEXACYANOMETALLATE ELECTRODE FOR ELECTROCHEMICAL APPLICATIONS, invented by Yuhao Lu et al., Ser. No. 13/523,694, filed Jun. 14, 2012, now U.S. Pat. No. 8,957,796, with an issue date of Feb. 17, 2015;

which is a Continuation-in-Part of a application entitled, ALKALI AND ALKALINE-EARTH ION BATTERIES WITH HEXACYANOMETALLATE CATHODE AND NON-METAL ANODE, invented by Yuhao Lu et al., Ser. No. 13/449,195, filed Apr. 17, 2012;

which is a Continuation-in-Part of a application entitled, ELECTRODE FORMING PROCESS FOR METAL-ION BATTERY WITH HEXACYANOMETALLATE ELECTRODE, invented by Yuhao Lu et al., Ser. No. 13/432,993, filed Mar. 28, 2012. All these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to electrochemical cells and, more particularly, to a transition-metal hexacyanoferrate (TMH) cathode battery and associated fabrication processes.

2. Description of the Related Art

A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. The energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential of −3.04 V vs. H₂/H⁺, and has the highest gravimetric capacity of 3860 milliamp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries have led the portable electronics revolution. However, the high cost of lithium metal renders doubtful the commercialization of lithium batteries as large scale energy storage devices. Further, the demand for lithium and its reserve as a mineral have raised the need to build other types metal-ion batteries as an alternative.

Lithium-ion (Li-ion) batteries employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. As a battery is cycled, lithium ions (Li⁺) are exchanged between the positive and negative electrodes. Li-ion batteries have been referred to as rocking chair batteries because the lithium ions “rock” hack and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO₂), or a material having a tunneled structure, such as lithium manganese oxide (LiMn₂O₄), on an aluminum current collector. The negative electrode (anode) material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge-discharge process, lithium ions are inserted into, or extracted from interstitial spaces of the active materials.

Similar to the lithium-ion batteries, metal-ion batteries use the metal-ion host compounds as their electrode materials in which metal-ions can move easily and reversibly. As for a Li⁺-ion, it has one of the smallest radii of all metal ions and is compatible with the interstitial spaces of many materials, such as the layered LiCoO₂, olivine-structured LiFePO₄, spinel-structured LiMn₂O₄, and so on. Other metal ions, such as Na⁺, K⁺, Mg²⁺, Al³⁺, Zn²⁺, etc., with large sizes, severely distort Li-based intercalation compounds and ruin their structures in several charge/discharge cycles. Therefore, new materials with large interstitial spaces would have to be used to host such metal-ions in a metal-ion battery.

Transition-metal hexacyanoferrates (TMHs) have been investigated as the cathode materials in lithium-ion batteries (LIBs) [1, 2] because they accommodate lithium-ion intercalation in their interstitial spaces. However, the lithium-ion size is too small to match the spaces, which degrades the TMH capacities rapidly during lithium-ion intercalation. In 2004, Eftekhari [3] used iron hexacyanoferrate (Prussian blue) as the cathode material in potassium-ion batteries (KIBs) with a counter electrode of potassium metal. The organic electrolyte was 1M KBF₄ in 3:7 ethylene carbonate/ethylmethyl carbonate (wt.). The size of potassium-ion is almost two times that of the lithium-ions, and matches the interstitial spaces of Prussian blue very well. The results showed that Prussian blue was a good electrode material for KIBs, demonstrating a reversible capacity of ca. 75 mAh/g and a good capacity retention.

Similarly, Cui's group studied the intercalation behavior of large ions, for example, sodium, potassium and ammonium ions, in copper (CuHCF) and nickel hexacyanoferrates (NiHCF) with an aqueous electrolyte [4-6]. These large size ions were compatible with the interstitial spaces of the hexacyanoferrates, so that CuHCF and NiHCF demonstrated good capacity retention. Due to the narrow electrochemical window of water, these materials were evaluated under low voltages and demonstrated low energy density. In order to improve the performance, organic electrolytes with a wide electrochemical window would have to be used to increase the operation voltages of the TMH electrodes.

Goodenough's group [7] investigated a series of Prussian blue analogues in sodium-ion batteries (SIBs) with organic electrolytes. They found that KFe(II)Fe(III)(CN)₆ demonstrated the highest capacity of 95 mAh/g, and KMnFe(CN)₆, KNiFe(CN)₆, KCuFe(CN)₆, and KCoFe(CN)₆ had a capacity of 50˜70 mAh/g. In the first 20 cycles, the capacity retention of KFeFe(CN)₆ was higher than 97%.

FIG. 1 is a diagram depicting the crystal structure of a transition-metal hexacyanoferrate (TMH) in the form of A_(x)M1M2(CN)₆ (prior art). Thins have an open framework. The large tetrahedrally coordinated A sites can host alkali, alkaline ions (A_(x)), and H₂O molecules. The number of alkali or alkaline ions in the large cages of this crystallographically porous framework may vary from x=0 to x=2, depending on the valence of M1 and M2 that are metal ions. Of course, as the electrode materials in SIBs or KIBs, TMHs are expected to have two of Na⁺ or K⁺-ions in their interstitial spaces. Therefore, M1 and M2 with valences of +2 are selected in the synthesis process to produce (Na,K)₂M1M2(CN)₆. Moreover, M1 and M2 can be reversibly oxidized and reduced between the valences of +2 and +3 when Na⁺- or K⁺-ions are extracted/inserted from/into TMHs. When these TMHs are used as the electrode materials in SIBs or KIBs, it is hard to obtain very smooth and flat charge/discharge curves due to the fact that M1 and M2 have different chemical potentials, or occupy different spin states. For example, Na₂CoFe(CN)₆ exhibited two plateaus in its discharge curves at 3.78 V and 3.28 V vs. Na/Na⁺, that correspond to the reduction of Co³⁺ and Fe³⁺, respectively [8].

In order to obtain cheap electrode materials for batteries, manganese is a good choice for TMHs, e.g., (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O. Matsuda and Moritomo [9] synthesized a Na_(1.32)Mn[Fe(CN)₆].3.5H₂O film for a lithium-ion battery that showed three plateaus in its discharge curve, and two plateaus in its charge curve that could be explained by the reduction and oxidation of Mn and Fe in the material. In the battery, the materials showed a capacity of 128 mAh/g. In a sodium-ion battery, Goodenough's group [7] also reported multiple plateaus in its charge/discharge curves. The Mn-based TMHs demonstrated a capacity of 70 mAh/g. However, it would be better to have a single, rather than two voltage plateaus in the charge/discharge curves. A battery with a single plateau charge/discharge curve has a tighter (more uniform) charge/discharge voltage than a battery with multiple plateaus. A tighter charge/discharge voltage renders a simpler battery control.

FIGS. 11A and 11B are graphs depicting the electrochemical behavior of a synthesized Na_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode in sodium-ion batteries (prior art). When sodium ions electrochemically move in and out of the interstitial space of magnesium hexacyanoferrate (MnHCF), two main potentials appear in the charge or discharge process, due to the redox reaction of Mn and Fe [7, 9] in MnHCF. In battery applications, the redox reaction of MnHCF causes two plateaus during charge/discharge. Two plateaus are observed during charge/discharge that correspond to the redox reaction of Mn at higher voltages and the redox reaction of Fe at low voltages.

-   [1] V. D. Neff, Some performance characteristics of a Prussian Blue     battery, Journal of Electrochemical Society, 132 (1985) 1382-1384. -   [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N.     Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron     cyanide complex as positive electrode of lithium secondary battery,     Journal of Power Sources, 79 (1999) 215-219. -   [3] Ali Eftekhari, Potassium secondary cell based on Prussian blue     cathode, Journal of Power Sources, 126 (2004) 221-228. -   [4] Colin D. Wessells, fibber A. Huggins, Yi Cui, Copper     hexacyanoferrate battery electrodes with long cycle life and high     power, Nature Communication, 2(2011) 550. -   [5] Colin a Wessel's, Sandeep V. Peddada, Robert A. Huggins, Yi Cui,     Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium     and potassium ion batteries. Nano Letters, 11(2011) 5421-5425. -   [6] Colin D. Wessells, Sandeep V. Peddada, M. T. McDowell, Robert A.     Huggins, Yi Cui, The effect of insertion species on nanostructured     open framework hexacyanoferrate battery electrodes, Journal of     Electrochemical Society, 159 (2012) A98-A103. -   [7] Vuhao Lu, Long Wang, John B. Goodenough, Prussian blue: a new     framework for sodium batteries, Chemistry Communication,     48(2012)6544-6546. -   [8] J. F. Qian, M. Zou, Y. L. Cao, H. X. Yang, Na_(x)M_(y)Fe(CN)₆     (M═Fe, Co, Ni): A New class of cathode Materials for sodium Ion     batteries, Journal of Electrochemistry (Chinese), 18(2012)108-112. -   [9] T. Matsuda, Y. Moritomo, Thin film electrode of Prussian blue     analogue for Li-ion battery, Applied Physics Express, 4(2011)047101.

It would be advantageous if a TMH cathode battery could be made to operate with a single plateau charge and discharge curve.

SUMMARY OF THE INVENTION

Disclosed herein are processes to prepare transition-metal hexacyanoferrate (TMH), such as (Na,K)_(x)Mn[Fe(CN)₆]_(y) zH₂O for sodium-ion batteries (SIBs) or potassium-ion batteries (KIBs). Three factors are associated with the improved processes. Firstly, reductive agents may be added into the synthesis solution to protect Mn²⁺ and Fe²⁺ from oxidation, so that more Na⁺- or K⁺-ions can be kept in the interstitial spaces of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O. Secondly, the product may be vacuum-dried at a temperature range from 20° C. to 200° C., regardless of whether it has been previously dried in air. Lastly, electronic conductors, for example, carbonaceous materials, can be dispersed into the reaction solution to improve performance of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O as electrode materials in SIBS or KIBs. The process can obtain (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O with the following performance:

(1) Just single plateau appears the charge/discharge curves.

(2) The charge/discharge curves are smooth and flat.

(3) The material exhibits a high capacity, energy efficiency, and coulombic efficiency.

Accordingly, a TMH cathode battery is provided. The battery has a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode, where the A cations are either alkali or alkaline-earth cations, where x is in the range of 1 to 2, where y is in the range of 0.5 to 1, and where z is in the range of 0 to 3.5. The A_(x)Mn[Fe(CN)₆]_(y).zH₂O has a rhombohectral crystal structure with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential. The battery also has an electrolyte, and an anode made of an A metal, an A composite, or a material that can host A atoms. The electrolyte may be an organic solvent containing an A-atom salt. In one aspect, A is either sodium (Na) or potassium (K).

The battery has a single plateau charging curve, where a single plateau charging curve is defined as a constant charging voltage slope between 15% and 85% battery charge capacity. Likewise, the battery has a single plateau discharge curve, where a single plateau discharge curve is defined as a constant discharge voltage slope between 85% and 15% battery charge capacity.

Additional details of the above-described battery, battery cathode, battery fabrication, and battery usage are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting the crystal structure of a transition-metal hexacyanoferrate (TMH) in the form of A_(x)M1M2(CN)₆ (prior art).

FIG. 2A is a diagram depicting an A_(x)MnFe(CN)₆ crystal structure, as an example of a TMH battery cathode material.

FIG. 2B is a diagram depicting a rhombohedral crystal structure.

FIG. 3 is a partial cross-sectional view of a TMH cathode battery.

FIG. 4 is a diagram depicting charging and discharging curves associated with the battery of FIG. 3.

FIGS. 5A and 5B are X-ray diffraction. (XRD) patterns contrasting, respectively, (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in air, with vacuum drying.

FIGS. 6A and 6B are graphs contrasting the electrochemical behavior of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in different conditions.

FIGS. 7A and 7B respectively depict the charge/discharge profile of the composite of graphene oxide (or graphene) and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O, and a chronoamperogram of the composite of graphene oxide (or graphene) and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O at the second cycle.

FIG. 8 is a flowchart illustrating a method for synthesizing a TMH battery material.

FIG. 9 is a flowchart illustrating a method for fabricating a TMH battery cathode electrode.

FIG. 10 is a flowchart illustrating a method for using a TMH battery.

FIGS. 11A and 11B are graphs depicting the electrochemical. behavior of a synthesized Na_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode in sodium-ion batteries (prior art).

DETAILED DESCRIPTION

FIG. 2A is a diagram depicting an A_(x)MnFe(CN)₆ crystal structure, as an example of a TMH battery cathode material. The cathode comprises A_(x)Mn[Fe(CN)₆]_(y).zH₂O;

where A cations are alkali or alkaline-earth cations;

where x is in the range of 1 to 2;

where y is in the range of 0.5 to 1; and,

where z is in the range of 0 to 3.5.

The A_(x)MnFe[(CN)₆]_(y).zH₂O has a rhombohedral crystal structure with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential. In one aspect, the A cations may be either sodium (Na) or potassium (K).

FIG. 2B is a diagram depicting a rhombohedral crystal structure. This structure is just a little bit “twisted” from a cubic crystal structure (α=β=γ90°). Although the rhombohedral crystal structure cannot physically be seen under a microscope, it can be detected using X-ray diffraction, see FIGS. 5A and 5B.

FIG. 3 is a partial cross-sectional view of a TMH cathode battery. The battery 300 comprises a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode 302, where A cations are either alkali or alkaline-earth cations, where x is in the range of 1 to 2, where y is in the range of 0.5 to 1, and where z is in the range of 0 to 3.5, see FIG. 2A. The A_(x)Mn[Fe(CN)₆]_(y).zH₂O has a rhombohedral crystal structure, with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential. The battery 306 has an anode 304 made from an A metal, an A composite, or a material that can host A atoms. The battery 300 further comprises an electrolyte 306. The electrolyte may, for example, be an organic solvent containing A-atom salts. In one aspect, A is either Na or K.

FIG. 4 is a diagram depicting charging and discharging curves associated with the battery of FIG. 3. As shown, the battery has a single plateau charging curve, where a single plateau charging curve is defined as a constant charging voltage slope between 15% and 85% battery charge capacity. Likewise, the battery has a single plateau discharge curve, where a single plateau discharge curve is defined as a constant discharge voltage slope between 85% and 15% battery charge capacity. It should be understood that the 15% and 85% battery charge capacity values may vary depending upon a particular design, and that in some instances, the range may extend to less than 15% battery charge capacity, and/or greater than 85%. Alternatively stated, the battery has a single plateau charging/discharging curve, where a single plateau charging/discharging curve is defined by the derivative of charge capacity to voltage (dQ/dV) of a completed charging/discharging curve having only one peak. In one aspect, the above-mentioned single plateau curves are associated with a battery having a discharge capacity of greater than 90 milliamp hours per gram (mAh/g). Alternatively stated, when there are two A atoms in a lattice (x=2), and all TMH materials are active (meaning they can charge and discharge), the discharge capacity associated with the first A atom is 90 mAh/g capacity. The discharge of the second A atom in the lattice increases the capacity to a value greater than 90 mAh/g.

As explained above, sodium or potassium (Na,K)_(x) Mn[Fe(CN)₆]_(y).zH₂O can be fabricated for sodium-ion batteries (SIBs) or potassium-ion batteries (KIBs) that demonstrate a single plateau during charge/discharge. In addition, these batteries have a high capacity. The preparation described herein (1) increases the value of x in (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O to obtain high capacity; (2) reduces the value of z in (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O; and, (3) enhances the electronic conductivity of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O.

Precipitation is a simple method to synthesize (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O. A Mn²⁺ solution and a Fe(CN)₆ ⁴⁻ solution are mixed, and (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O precipitates immediately. When the process is exposed to air, it is inevitable that Mn²⁺- or Fe²⁺-ions are oxidized to Mn³⁺ or Fe³⁺. The oxidation reduces the content of Na⁺ or K⁺ in the final product of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O because all charges should be neutralized in the material. As electrode materials, the small content of Na⁺ or K⁺ means a small capacity of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O for SIBs or KIBs. In order to prevent the oxidation in the synthesis, reductive agents can be added into both the Mn²⁺ solution, the Fe(CN)₆ ⁴⁻ solution, or both, at the beginning of the process. The reductive agents may, for example, be sodium borohydride (NaBH₄) and hydrazine (N₂H₄), but other agents would be apparent to those with skill in the art. In the solution of Mn²⁺, the concentration of reductive agents is from 0 to 100 moles/liter. In the solution of Fe(CN)₆ ⁴⁻, the concentration of reductive agents is from 0 to 100 in moles/liter. After the reaction finishes, the precipitate is collected and washed to obtain (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O.

FIGS. 5A and 5B are X-ray diffraction (XRD) patterns contrasting, respectively, (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in air, with vacuum drying. The XRD pattern of FIG. 5A is associated with a cubic crystal structure and the XRD pattern of FIG. 5B is associated with a rhombohedral crystal pattern. (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O can be dried in either air or in a vacuum. The crystal structure and performance of the (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O is highly dependent upon drying condition. Na_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in air (FIG. 5A) has a cubic structure, but it shows a structure change to a rhombohedral phase after being dried in a vacuum with a pressure of less than 0.1 torr (FIG. 5B).

FIGS. 6A and 6B are graphs contrasting the electrochemical behavior of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in different conditions. A Na_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode was evaluated in standard CR2032 coin cells that consisted of a sodium anode or hard carbon anode, separated by a Celgard polypropylene separator. The Na_(x)Mn[Fe(CN)₆]_(y).zH₂O electrode was made of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O, carbon black (Super P), and polytetrafluoroethylene (PTFE) binder. The electrolyte was saturated NaClO₄ in 1:1 ethylene carbonate/diethyl carbonate (EC/DEC) (vol.:vol.). The charge/discharge currents were 0.1 C @ 170 milliamp-hours per gram. (mAh/g). All cells were cycled between 2 V and 4.2 V.

After being dried in air, the Na_(x)Mn[Fe(CN)₆]_(y).zH₂O shows two plateaus during charge and discharge (FIG. 6A). However, only one plateau is observed in the charge/discharge curves after vacuum drying with a pressure of less than 0.1 torr (FIG. 6B). Moreover, the vacuum drying process increases the energy density and efficiency of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O. The fact that a material with the same chemical formula of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O may demonstrate different crystal structures and different charge/discharge, profiles is a result of different drying conditions.

In order to improve the performance of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O in SIBs or KIPS further, manganese TMHs can be composited with electronic conductors. In the composite structure, small sized (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O particles can be uniformly coated on the large surface area of the conductors, so that the electronic conductivity of the (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O electrode is improved, and the utilization rate of the active (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O material is improved. The high conductivity, uniform distribution of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O, and high utilization rate improves battery performance in terms of capacity and power. The electronic conductor may be metal powders, carbonaceous materials, and polymers, but is not limited to just these examples. With the vacuum drying process, a (Na,K)^(x)Mn[Fe(CN)₆]_(y).zH₂O conductor composite structured electrode shows a single plateau during charge and discharge. Moreover, its capacity is improved significantly due to the higher utilization rate.

A composite of graphene and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O, as an example, is also useful in processing. Graphene oxide is ultrasonically dispersed into distilled water. The graphene oxide solution can be put into the Mn²⁺ solution, the Fe(CN)₆ ⁴⁻ solution, or both. Using the precipitation method with reductive agent, the graphene oxide is graphene and a composite of graphene and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O forms. After separation and washing, the composite is dried in an oven at a temperature between 20° C. and 200° C. The final product is a composite Na_(x)Mn[Fe(CN)₆]_(y).zH₂O/graphene powder. If the drying is in an vacuum oven with pressure less than 0.1 torr, the Na_(x)Mn[Fe(CN)₆]_(y).zH₂O has a rhombohedral crystal structure.

FIGS. 7A and 7B respectively depict the charge/discharge profile of the composite of graphene oxide (or graphene) and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O, and a chronoamperogram of the composite of graphene oxide (or graphene) and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O at the second cycle. A vacuum drying process was applied to the composite Na_(x)Mn[Fe(CN)₆]_(y).zH₂O/graphene powder. The counter electrode is sodium metal and the organic electrolyte is saturated NaClO₄ in EC/DEC. Graphene oxide in the synthesis solution increases the capacity of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O significantly. The vacuum drying process results in the Na_(x)Mn[Fe(CN)₆]_(y).zH₂O having single and very flat charge/discharge curves (FIG. 7A), indicating that sodium-ion intercalation in the electrode is mostly likely a two-phase process, rather than the solid solution behavior shown in FIG. 6A. The electrode's chronoamperogram. (FIG. 7B) also demonstrates the two-phase process with a single sharp peak during charge/discharge.

FIG. 8 is a flowchart illustrating a method for synthesizing a TMH battery material. Although the method is depicted as a sequence of numbered steps for clarity, the numbering does not necessarily dictate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. Generally however, the method follows the numeric order of the depicted steps. The method starts at Step 800.

Step 802 prepares a first solution of Mn²⁺. Step 804 prepares a second solution of an A salt of Fe(CN)₆ ⁴⁻. Step 806 mixes the first and second solutions. In response to mixing the first and second solutions, A_(x)Mn[Fe(CN)₆]_(y).zH₂O is precipitated in Step 808. The A cations are alkali or alkaline-earth cations, such as Na or K. The variable x is in the range of 1 to 2, y is in the range of 0.5 to 1, and z is in the range of 0 to 3.5. Step 810 dries the precipitated A_(x)Mn[Fe(CN)₆]_(y).zH₂O in a vacuum oven with a pressure of less than 0.1 torr. In some aspect, the pressure is less than 0.01 torr. For example, the drying temperature may be in the range of 20° C. to 200° C. In one aspect, Step 808 forms the A_(x)Mn[Fe(CN)₆]_(y).zH₂O in a cubic structure and Step 812 forms the A_(x)Mn[Fe(CN)₆]_(y).zH₂O with a rhombohedral crystal structure, with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.

During the synthesis process, it is inevitable that water molecules occupy the large interstitial spaces of TMHs. Water molecules in TMHs cause the A ions to move from the center of the elementary cubes, which makes the A ions interact with the two transition-metal redox couples differently. That is, the A-ions are located in two chemical environments. The charge/discharge, or extraction/insertion of A ions in two chemical environments correspond to the two charge/discharge plateaus seen in FIG. 11A. Under a vacuum with less than 0.1 torr pressure, water molecules are removed from the interstitial spaces, permitting A-ions go to the center of the cubes. All A-ions then have a single, identical chemical environment, which causes just one plateau during charge/discharge. Further, the cubic structure of TMHs is changed to rhombohedral due to the water removal. It is still possible for rhombohedral TMHs to exhibit two plateaus during charge/discharge if the A-ions do not occupy the center positions due to the water molecules in TMHs. However, the water removal from the crystal results in a phase transformation from cubic to rhombohedral, and also is the root cause of the one-plateau behavior of TMHs described herein. In other words, the use of higher vacuum pressure results in the more effective removal of water from the TMH crystal structure, and the single plateau charge/discharge curves.

In one aspect, prior to mixing the first and second solutions in Step 806, Step 805 b adds reductive agents to the first solution, the second solution, or both the first and second solutions. Some exemplary reductive agents include sodium borohydride (NaBH₄), hydrazine (N₂H₄), and a combination of NaBH₄ and N₂H₄. For example, Step 805 b may add the reductive agents to the first solution in a concentration in a range of 0 to 100 moles/liter. Likewise, the reductive agents may be added to the second solution in a concentration of 0 to 100 moles/liter.

In a different aspect, Step 805 a ultrasonically disperses carbonaceous materials in distilled water, creating a third solution. Then, Step 806 additionally mixes the third solution with the first solution, second solution, or both the first and second solutions, and precipitating the A_(x)Mn[Fe(CN)₆]_(y).zH₂O in Step 808 includes precipitating a composite of carbonaceous materials and A_(x)Mn[Fe(CN)₆]_(y).zH₂O. Some examples of carbonaceous materials include graphene oxide, partially reduced graphene oxide, graphene, carbon black, and graphite. If the carbonaceous material used in Step 805 a is graphene oxide, then Step 805 b may be performed by adding a reducing agent such as NaBH₄, N₂H₄, or both NaBH₄ and N₂H₄ to the mixture of the first, second, and third solutions.

FIG. 9 is a flowchart illustrating a method for fabricating a TMH battery cathode electrode. The method begins at Step 900. Step 902 mixes material with a conducting carbon and an organic binder in an organic solution, creating a A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste. The A cations are alkali or alkaline-earth cations, such as Na and K. The variable x is in the range of 1 to 2, y is in the range of 0.5 to 1, and z is in the range of 0 to 3.5. Step 904 forms A_(x)Mn[Fe(CN)₆]_(y).zH₂O material on a current collector to create an electrode. The current collector may, for example, be aluminum (Al), titanium (Ti), or stainless steel. Step 906 dries the electrode. Step 908 supplies the A_(x)Mn[Fe(CN)₆]_(y).zH₂O electrode with a rhombohedral crystal structure, with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.

In one aspect, the material used in Step 902 is the end product (Step 812) of the method described in FIG. 8, in which case TMH material has already been vacuum dried at a pressure of less than 0.1 torr. Alternatively, creating the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste in Step 902 includes creating the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste in a cubic structure, and Step 906 dries the electrode in a vacuum oven at a pressure of less than 0.1 torr.

In one aspect, forming the A_(x)Mn[Fe(CN)₆]_(y).zH₂O material on the current collector in Step 904 includes substeps. Step 904 a coats the current collector with the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste. Step 904 b dries the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste, and Step 904 c presses the coated current collector. After printing and drying (Steps 904 a and 904 b), the electrode is very porous due to solvent evaporation. The particles (TMH and conducting carbon) are bonded by organic binder. However, for a battery, the ideal porosity for electrode is smaller, but still large enough to permit the entry of the electrolyte into the electrode. The porosity is reduced by pressing is to compact the particles assembly.

In another aspect, forming the A_(x)Mn[Fe(CN)₆]_(y).zH₂O material on the current collector in Step 904 includes alternative substeps. Step 904 d forms a self-standing film from the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste. Step 904 e presses the self-standing film on the current collector.

FIG. 10 is a flowchart illustrating a method for using a TMH battery. The method begins at Step 1000. Step 1002 provides a battery with an anode, an electrolyte, and a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode. The A cations are alkali or alkaline-earth cations, such as Na or K. The variable x is in the range of 1 to 2, y is in the range of 0.5 to 1.0, and z is in the range of 0 to 3.5. The electrolyte may be an organic solvent containing an A-atom salt. Step 1004 connects a load between the cathode and the anode. Step 1006 discharges the battery in a single plateau discharge curve, where a single plateau discharge curve is defined as a constant discharge voltage slope between 85% and 15% battery charge capacity. Alternatively stated, in Step 1006 the single plateau discharging curve is defined by the derivative of charge capacity to voltage (dQ/dV) of a completed discharging curve having only one peak. In one aspect, Step 1006 discharges with a capacity of greater than 90 milliamp-hours per gram (mAh/g).

Subsequent to disconnecting the load in Step 1008, Step 1010 connects a battery charging device between the cathode and the anode. Step 1012 charges the battery in a single plateau charge curve, where a single plateau charge curve is defined as a constant charge voltage slope between 85% and 15% battery charge capacity. Alternatively stated, the single plateau charging curve is defined by the dQ/dV of a completed charging curve having only one peak. The battery can be iteratively charged and discharged. In one aspect not shown, the battery can be charged while still connected to a load.

In one aspect, Step 1002 provides the battery cathode A_(x)MnFe(CN)₆.zH₂O having a rhombohedral crystal structure with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.

A transition metal hexacyanoferrate (TMH) battery, TMH battery fabrication, and TMH battery usage has been provided. Examples of particular materials and process steps have been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art. 

We claim:
 1. A transition metal hexacyanoferrate (TMH) battery cathode comprising: A_(x)Mn[Fe(CN)₆]_(y) .zH₂O; where A cations are selected from a group consisting of alkali and alkaline-earth cations; where x is in a range of 1 to 2; where y is in a range of 0.5 to 1; where z is in a range of 0 to 3.5; and, where the A_(x)MnFe[(CN)₆]_(y).zH₂O has a rhombohedral crystal structure with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.
 2. The TMH battery cathode of claim 1 wherein the A cations are selected from a group consisting of sodium (Na) and potassium (K).
 3. A transition metal hexacyanoferrate (TMH) cathode battery, the battery comprising: a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode; where A cations are selected from a group consisting of alkali and alkaline-earth cations; where x is in a range of 1 to 2; where y is in a range of 0.5 to 1; where z is in a range of 0 to 3.5; where the A_(x)Mn[Fe(CN)₆]_(y).zH₂O has a rhombohedral crystal structure with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential; an anode selected from a group consisting of an A metal, an A composite, and a material that can host A atoms; and, an electrolyte.
 4. The battery of claim 3 wherein the electrolyte is an organic solvent containing A-atom salts.
 5. The battery of claim 3 wherein A is selected from a group consisting of sodium (Na) and potassium (K).
 6. The battery of claim 3 wherein the battery has a single plateau charging curve, where a single plateau charging curve is defined as a constant charging voltage slope between 15% and 85% battery charge capacity.
 7. The battery of claim 3 wherein the battery has a single plateau discharge curve, where a single plateau discharge curve is defined as a constant discharge voltage slope between 85% and 15% battery charge capacity.
 8. The battery of claim 3 wherein the battery has a single plateau charging/discharging curve, where a single plateau charging/discharging curve is defined by a derivative of charge capacity to voltage (dQ/dV) of a completed charging/discharging curve having only one peak.
 9. The battery of claim 3 wherein the battery has a discharge capacity of greater than 90 milliamp hours per gram (mAh/g).
 10. The battery of claim 3 wherein the anode comprises a primary material selected from a group consisting of hard carbon and graphite. 